In the usual course of things back then the "kit engineer" took the designer's plans/sketches/measurements and used that info to design the kit plane. The designer usually would get some say in the changes, like Riley Wooten posted a bit ago- Carl Goldberg made some changes to the VooDoo kit to make sure it would be flyable by less experienced flyers. They also had to fit it into economical packaging. Even those paste board boxes back then were a significant part of the cost because they had to be bought in relatively large quantities to keep the price down.
Phil C